1. Field of the Invention
This invention relates generally to microwave-frequency oscillators, and relates more particularly to a YIG tuned oscillator having both series and parallel feedback.
2. Description of the Relevant Art
Prior wideband, microwave-frequency oscillators have used yttrium-iron-garnet (YIG) spheres as resonant elements and transistors, either bipolar or field-effect, as active elements. These oscillators are commonly known as YIG-tuned oscillators (YTO). One or more reactive elements coupled to the transistor establish the conditions for oscillation. The YIG sphere is placed in a magnetic field and resonates in accordance with the strength of that magnetic field. Such an oscillator is tuned by varying the magnetic field applied to the YIG, thus changing the resonant frequency of the YIG and the frequency of the oscillator. The YIG sphere is RF-coupled to the transistor, forming either a series or parallel feedback element.
In oscillators using the YIG as a series feedback element, the oscillator makes use of the reflection gain (S.sub.11) of the transistor element to establish conditions for oscillation. A typical series feedback YIG oscillator, using a common-base bipolar transistor as the active element, would couple an inductor to the base of the transistor as a reactive element and would RF couple the YIG sphere to the emitter of the transistor. A shunt capacitor and the output load would be connected to the collector of the transistor. To maximize tuning range, the base inductance and collector shunt capacitance would be selected for achieving negative resistance in the widest band possible. This design is limited by the difficult in achieving negative resistance at both the low and high end of the frequency band.
On the other hand, oscillators using the YIG as a parallel feedback element use the transmission gain (S.sub.21) of the transistor to power the oscillator. A typical parallel feedback oscillator, again using a common emitter bipolar transistor as the active element, would couple the YIG between the collector and the base of the transistor and would feedback to the base part of the output power supplied by the collector. Since the YIG in this configuration would provide reactive feedback, no reactive component apart from the YIG is needed. Oscillators of this type typically operate at lower frequencies than series feedback oscillators.